1. Field of the Invention
The present invention relates to vehicle automatic transmissions in which both a first speed clutch and a first speed ho d clutch are provided side by side.
2. Description of the Prior Art
Japanese Patent Application Laid-open No. 62-141343 discloses a continuously variable transmission for a vehicle comprising two clutches, and provides a transmission in which a pair of clutch pistons are fitted in a relatively movable manner, one of the clutches being engaged by movement of the outside clutch piston and the other clutch being engaged by relative movement of the inside clutch piston against the outside clutch piston.
In the above-mentioned prior art device, since no special consideration is given to the order in which the hydraulic pressure is applied to the two clutch oil chambers when both clutches are engaged, the operation of the two clutches is not in order and there are the problems that pulsations are caused in the transmitted torque and in-gear shock occurs. Moreover, since the supply and discharge of hydraulic fluid to and from the clutch oil chamber of one of the clutches is carried out via a single oil passage only provided inside the rotational shaft, it is difficult to discharge the hydraulic fluid from the clutch oil chamber when the engagement of one of the clutches is released and thus there is the problem that the responsiveness of the clutch disengagement is lowered, particular at low temperatures where the viscosity of the hydraulic fluid increases.
The present invention has been carried out of the above-mentioned circumstances and is directed to an automatic transmission comprising a first speed clutch and a first speed old clutch side by side. It is an object of the present invention to smoothly establish a first speed hold gear shift stage and enhance the responsiveness of disengagement of the first speed clutch, particularly at low temperatures.
In order to achieve the above-mentioned object, a vehicle automatic transmission comprises a first speed clutch for establishing a first speed gear shift stage when it is engaged, a first speed hold clutch for establishing a first speed hold gear shift stage when it is engaged, a first clutch piston for causing frictional engagement elements of the first speed clutch to be engaged with each other, and a first clutch oil chamber for driving the first clutch piston in a direction to make the first speed clutch engaged. A second clutch piston is installed inside the first clutch piston in a relatively movable manner and causes the frictional engagement elements of the fist speed hold clutch to be engaged with each other, a second clutch oil chamber drives the second clutch piston in a direction to make the first speed hold ditch engaged, and a clutch housing is shared by the first speed clutch and the first speed hold clutch. A check valve is provided on the first clutch piston which discharges hydraulic fluid from the first clutch oil chamber into the inside of the clutch housing when the hydraulic pressure of the first clutch oil chamber is less than a predetermined value and cuts off the connection between the first clutch oil chamber and the inside of the clutch housing when the hydraulic pressure of the first clutch oil chamber is at or above the predetermined level. Oil outlets are formed in the clutch housing which connect the outside and the inside of the clutch housing, when a first speed hold gear shift stage is established, after the first speed clutch has been engaged by supplying hydraulic fluid to the first clutch oil chamber, so as to drive the first clutch piston in the direction of engagement. The first speed hold clutch is engaged by supplying hydraulic fluid to the second clutch oil chamber to drive the second clutch piston relative to the first clutch piston in the direction of engagement.
In accordance with the above-mentioned arrangement, when a first speed hold gear shift stage is established, since the first speed hold clutch is engaged by supplying hydraulic fluid to the second clutch oil chamber after the first speed clutch has been engaged by supplying hydraulic fluid to the first clutch oil chamber, the speed at which the first clutch piston of the first speed clutch provides engagement and the speed at which the second clutch piston of the first speed hold clutch provides engagement, can be appropriately controlled to prevent pulsations in the transmitted torque and the occurrence of in-gear shock.
Moreover, since oil outlets are formed in the clutch housing to link the inside and the outside of the clutch housing, when the hydraulic pressure of the first clutch oil chamber of the first speed clutch is released in order to cancel the first speed hold gear shift stage, the hydraulic fluid which has been discharged from the first clutch oil chamber into the inside of the clutch housing accompanying the opening of the check can be discharged via the oil outlets from the inside of the clutch housing. Thus, even at low temperatures where the viscosity of the hydraulic fluid increases, the hydraulic fluid can be rapidly discharged from the first clutch oil chamber so as to enhance the responsiveness of disengagement of the first speed clutch.